The invention relates to a method of producing open-pored sintered glass with special characteristics that allow its application as a filter for fluid or gaseous media.
Shaped parts composed of open-pored sintered glass with pore volumes of 50%-85% are produced according to conventional procedures. With pore volumes of more than 60%, however, flexural tensile strengths of only about 2 N/mm.sup.2 result, which are too low for application as a filtering medium because the maximum applicable pressure differential would have to be considerably smaller than 1 bar. A possible solution to this problem might lie in making the filter thicker, which would, however, occur at the expense of the flow velocity.
The characteristics of the open-pored sintered glass with pore volumes over 60%, which are not advantageous for filtration, relate to two causes:
1. The distribution of pore sizes is relatively widely scattered around an average value. The flow velocity for a fluid or gaseous medium is mainly determined by the largest pore diameters, while the smaller pores contribute only a slight amount to this on the basis of the Hagen-Poiseuille Law. On the other hand, the pore volume formed by small pores diminishes the flexural tensile strength. FIG. 1 shows pore size distribution with a conventional sintered glass sample as measured by the mercurypenetration method. Other characteristics of this sintered glass body: Average pore diameter 27 .mu.m, flexural tensile strength 1.8 N/mm.sup.2, flow rate for water 16 ml/cm.sup.2.s, pore volumes 74%.
2. Open-pore sintered glass produced according to the conventional manner has pores with highly structured inner surface areas, as the rastered electron-microscopic micrographs in FIG. 2 show. This high degree of structuring retards the free passage of flowing media. The retained filtration residues can be removed only with great difficulty. Furthermore, the illustrated structures can be the starting point for tears, which lower the flexural tensile strength.
The commercial laboratory filters which are composed of borosilicate glass 3.3 also have a highly structured inner surface, as the rastered electronmicroscopic micrograph in FIG. 3 shows, a broken-edge of a currently commercial laboratory filter composed of borosilicate glass 3.3 (DIN ISO 3585, 650 X magnification) is illustrated. Such filters are produced by means of sintering glass powder without the addition of a flux. The individual glass particles are still clearly recognizable as they existed prior to sintering. Such structures make the passage of flowing media and clean-up after use more difficult.
Additionally, such filters can only be produced with pore volumes of at most 50%; their pore radii have more widely scattered distributions than the sintered glass represented in FIG. 1.
A procedure for the production of porous sintered glass is known from U.S. Pat. No. 4,588,540, by which glass powder is mixed with a readily soluble substance and the mixture is heated to the sintering temperature of the glass and maintained there until the glass powder is sintered, after which the product is cooled and the readily soluble substance is dissolved from it. The pore size distribution resulting hereby is similar to the one represented in FIG. 1.